Current best practice (subject to change) is to use the following approach in your CSS file for your responsively designed projects:
// main.css
body {
// default font-size for most browsers is 16px
// setting it to 100% ensures that the base size
// will conform to the browser defaults
font-size: 100%;
}
.content {
// use ems or rems for all other font-sizes
font-size: 1.2em;
}